It is known to produce an internal latent image type core/shell direct positive silver halide emulsion as described below.
In Japanese Patent Application (OPI) 136641/82 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), a process for producing an internal latent image type core/shell emulsion useful as a direct emulsion is described. The process comprises carrying out chemical ripening of the surface of core/shell type silver halide grains in an internal latent image type emulsion in the presence of a polymer such as poly(N-vinylpyrrolidone), poly(N-vinyloxazolidone), a vinyl alcohol-N-vinylpyrrolidone copolymer or an N-vinylpyrrolidone-vinyl acetate copolymer, where the core/shell type silver halide grains are obtained by precipitating an outer shell (shell) of silver halide to cover at least light-sensitive sites on an internal nucleus particle (core particle) of silver halide subjected to doping with a metal ion, chemical sensitization or both.
In Japanese Patent Publication 34213/77 (U.S. Pat. No. 3,761,276, Evans), an internal latent image type emulsion useful as a direct positive emulsion is described. This emulsion is characterized by the fact that a doping agent is contained in the inner part of silver halide grains and the surface of the grains is chemically sensitized. The same type of emulsion is also disclosed in U.S. Pat. No. 3,317,322, Porter et al.
On the other hand, in the case of preparing the above described internal latent image type core/shell direct positive silver halide emulsions, it is known that internal latent image type core-shell silver halide emulsions having good reversal performance which are suitable for the desired purpose can be produced using various kinds of photographic additives.
For example, silver halide grains having a large particle size and a narrow particle size distribution (namely, uniform particle size) are obtained by adding a sulfur containing compound such as a thione compound as described in Japanese Patent Application (OPI) 82408/78 or 144319/78 or a thioether compound as described in U.S. Pat. No. 3,574,628 during the formation of the silver halide grains. These compounds are known as silver halide solvents and have the advantage that a hard tone high speed direct positive emulsion is obtained by addition in the case of forming the internal nucleus particles (core) of the internal latent image type core/shell silver halide grains. Further, there is the advantage that the light-sensitive sites of the core particles can be well covered in a very short time, if the above described sulfur containing compound is added in the case of precipitating the outer shell (shell) of the silver halide on the core particles.
Further, silver halide grains having a desired crystal form or particle size can be produced by adding a dye in the case of forming silver halide grains as described in Japanese Patent Application (OPI) 26589/80, and direct positive emulsions having a desired crystal form are obtained by adding the dye in the case of forming the core particles of internal latent image type core/shell silver halide grains or in the case of precipitating the shell on the core particles.
When a sulfur containing compound which is strongly adsorbed on the surface of silver halide grains to restrain the growth thereof (hereinafter referred to as a "sulfur containing restrainer") such as mercaptotetrazoles, mercaptotriazoles, mercaptothiazoles or benzothiazole-2-thiones, etc., is added in the case of the formation of the core particles of internal latent image type core/shell silver halide grains, fine silver halide grains having a comparatively uniform particle size can be obtained. The sulfur containing compounds, including the above described sulfur containing silver halide solvents, permit large silver halide grains having a comparatively uniform particle size to be obtained and have the advantage that the particle size of the core particles can be easily controlled at will. Further, when the above described sulfur containing restrainer is used in the case of the formation of the core particles or in the case of the precipitation of the shell, silver chloride (regular octahedral or rhombododecahedral) and silver iodobromide (rhombododecahedral) having a singular crystal form which is difficult to obtain by conventional processes can be sometimes obtained, which particles have high utility for various purposes. Moreover, it has been known that when a thione compound as described in Japanese Patent Application (OPI) 29829/80 is used in the case of carrying out chemical sensitization of the core particles of internal latent image type core/shell silver halide grains, the function of a gold sensitizer is effectively promoted to obtain a high speed direct positive emulsion.
These sulfur containing compounds (sulfur containing silver halide solvents, sulfur containing restrainers and sulfur containing sensitizing assistants) and dyes act as effective photographic additives, respectively, in each step in the case of preparing an internal latent image type core/shell silver halide emulsion. The preparation of an internal latent image type core/shell silver halide emulsion is generally roughly divided in four steps, namely: (1) formation of core particles, where the core particles may be doped with metal ion; (2) chemical sensitization of core particles; (3) precipitation of a shell on the core particles; and (4) surface chemical ripening of core/shell silver halide grains. However, they frequently have a harmful influence on subsequent steps. For example, a sulfur containing restrainer used in the case of the formation of core particles in step (1) remarkably restrains reaction in the case of chemical sensitization of the core particles in step (2), or a silver halide solvent used in step (1) or (3) puts the chemical ripening centers formed by surface chemical ripening of the core/shell emulsion in step (4) into the inner part of the grains so as not to form effective surface chemical sensitization centers, or a sulfur containing compound or a dye used in steps (1) to (4) remains on the surface of the core/shell silver halide grains after completion of step (4) to obstruct adsorption of spectral sensitizers, various photographic stabilizers (for example, hydroxytetraazaindenes, etc.) or nucleating agents, etc. Such problems are also observed in conventional surface latent image type silver halide emulsions for forming negative images. However, since preparation of internal latent image type core/shell silver halide emulsions generally comprises four steps as described above, it has been greatly desired that photographic additives effective in each step do not have any influence upon the subsequent steps.
Some examples of characteristic of internal latent image type core/shell silver halides are described in the following. The character of the surface chemical ripening of internal latent image type core/shell silver halide emulsions is fairly different from surface chemical ripening of conventional surface latent image type silver halide emulsions, and a required aspect is to make the surface sensitivity of the core/shell emulsion remarkably low as compared with the internal sensitivity of the core/shell emulsion so that latent images are formed in the inner part of the grains. Accordingly, surface chemical ripening of the core/shell silver halide grains should be weak as compared with surface chemical sensitization of surface latent image type silver halide grains and, consequently, surface chemical ripening of the core/shell silver halide grains is easily affected by residual sulfur containing compound or dye.
In core/shell silver halide grains, chemically ripening the surface of said grains is carried out in order to increase the maximum density (D.sub.max) of the reversal image obtained by direct reversal processing. If the above described sulfur containing compound or dye is present in the case of carrying out surface chemical ripening, the maximum density obtained (D.sub.max) becomes low or saturation of the maximum density (D.sub.max) requires a long time because the surface sensitive centers formed are buried or surface chemical ripening is remarkably retarded. As means for increasing maximum density (D.sub.max), there is one process where the degree of surface chemical sensitization is slightly enhanced. However, if the surface chemical sensitization is excessively carried out (beyond the optimum range), the minimum density (D.sub.min) of the reversal images increases or the sensitivity of re-reversal images (negative images) increases, and good reversal performance cannot be obtained.
The fact that saturation of maximum density (D.sub.max) requires a long time means that the reversal performance (particularly, D.sub.max) change due to a small change in the time for surface chemical ripening is great, which is disadvantageous from the viewpoint of producing the internal latent image type core/shell silver halide emulsions in a stable manner.
Further, since the surface chemical ripening centers of the internal latent image type cores/shell emulsions are weak, photographic properties are easily affected by various additives (for example, spectral sensitizers or stabilizers; hereafter referred to as "finish additive(s)") added before application to a base after production of the core/shell emulsion. These finish additives are adsorbed on the surface of the core/shell silver halide grains to cause a desired effect. However, if a sulfur containing compound or dye used in the preparation of the core/shell silver halide emulsion remains adsorbed on the surface of the silver halide grains in the case of adding the finishing additives to the core/shell silver halide emulsion, they are adsorbed on the surface of silver halide grains in competition with the compound or due and, consequently, adsorption is unstable and stability with the passage of time is inferior. Particularly, in core/shell silver halide grains as described above, photographic properties (D.sub.max, D.sub.min, reversal image sensitivity and re-reversal image sensitivity) are greatly changed by adsorption of the finish additives, if adsorption of the finish additives is unstable or changes with the passage of time, and photographic properties unpredictably change. Therefore, there is a serious problem from the viewpoint of providing stabilized coated light-sensitive materials.
In the production of internal latent image type core/shell silver halide emulsions, it has been highly desired that, after photographic additives (particularly, sulfur containing compounds or dues) are effectively used in each step, they have no influence on the next step. Sulfur containing compounds and dues as described above are very strongly adsorbed on silver halide grains and are not desorbed by conventional desorption processes such as a reduction of pH or an increase of pAg, and many of them cannot easily be removed by washing the emulsion with water. Accordingly, it has been desired to develop a novel process for removing sulfur containing compounds or dyes.
Further, in Japanese Patent Application (OPI) 66727/78, a special process for producing internal latent image type direct positive silver halide emulsions is disclosed. Specifically, it is proposed that re-reversal negative images in high illumination exposure areas which are caused in an internal latent image type emulsion containing 10 mol % or less of silver iodide, which is produced by reacting at least 80 mol % based on the total amount of water soluble iodide with silver salt after formation of 1/4 the molar amount based on the total amount of silver halide, in a step of forming silver halide grains by reacting a silver salt with a water soluble halide, can be prevented by adding an oxidizing agent after production of the emulsion but before addition of couplers, etc.
In Japanese Patent Application (OPI) 70221/83, a process where "acid-processed gelatin" is used as the gelatin added in the case of setting after production of an internal latent image type direct positive silver halide emulsion is proposed. This process is described as having the effect that photographic performance does not deteriorate during setting and preservation of the emulsion.
In internal latent image type core/shell direct positive silver halide emulsions, there is a tendency that re-reversal images are formed to a great extent with increases in sensitivity or maximum density (D.sub.max), and there is the tendency that storage stability or production stability is deteriorated.
Particularly, in the production of practical emulsions to which various kinds of photographic additives are added, it has been desired to achieve good production stability for the emulsion. Requirements for good production stability include, for example, the following.
(1) The core/shell silver halide exhibits rapid surface post-ripening (whereby D.sub.max is stabilized) ("rapid surface post-ripening" means that D.sub.max reaches saturation in the early stages of post-ripening).
(2) The core/shell silver halide emulsion has good dissolution stability with the passage of time (the term "good dissolution stability with the passage of time" means that the photographic performance of a finished coating solution (silver halide emulsion prepared so as to be applied to a base) after all desired additives are added remains unchanged with the passage of time. In this sense, it is different from "preservation stability" of the photographic light-sensitive material after the finished emulsion has been applied to a base.).
In the production of internal latent image type core/shell direct positive emulsions, it has been desired to overcome the above problems. However, satisfactory results have not been obtained by processes for producing emulsions as hereto proposed.